A non-reciprocal circuit device is a circuit device comprising a magnetic body of ferrite such as garnet, pluralities of crossing central conductors disposed on the magnetic body, and a magnet applying a DC magnetic field to the magnetic body to generate a rotating resonance magnetic field in the magnetic body, thereby transmitting signals input to one central conductor to another central conductor without attenuation.
FIG. 12 shows the equivalent circuit of a non-reciprocal circuit device called “two-port isolator,” which is disclosed in JP 2004-15430 A, and FIG. 13 shows the structure of this non-reciprocal circuit device. This two-port isolator comprises a first input/output port P1, a second input/output port P2, a first inductance element Lin and a first matching capacitor Ci connected between the input/output ports P1, P2 for constituting a first parallel resonance circuit, a resistance element R parallel-connected to the first parallel resonance circuit, and a second inductance element Lout and a second matching capacitor Cf connected between the second input/output port P2 and the ground for constituting a second parallel resonance circuit. The feature of the two-port isolator is that the first parallel resonance circuit determines a frequency at which isolation (opposite-direction attenuation) is maximum, while the second parallel resonance circuit determines a frequency at which insertion loss is minimum.
As shown in FIG. 13, the first inductance element Lin and the second inductance element Lout are in a strip shape constituted by the first central conductor Lin and the second central conductor Lout, crossing with insulation on or in a ferrite plate, to which a DC magnetic field is applied from a permanent magnet 30, to constitute a central conductor assembly 4. The first matching capacitor Ci and the second matching capacitor Cf are formed by electrode patterns in the multilayer ceramic substrate 10. A main surface of the multilayer ceramic substrate 10 is provided with an electrode pad 15 and connecting pads 17, 18. The electrode pad 15 is connected to a terminal electrode P2 of the second central conductor Lout formed on a side surface of the multilayer ceramic substrate 10 through via-holes electrode and side-surface electrodes. The connecting pad 17 is connected to a terminal electrode P1 of the first central conductor Lin formed on a side surface of the multilayer ceramic substrate 10 through via-holes electrode and side-surface electrodes. The connecting pad 18 is connected to a ground electrode GND through via-holes electrode and side-surface electrodes. The permanent magnet 30, the central conductor assembly 4 and the multilayer ceramic substrate 10 are contained in upper and lower cases 22, 25 made of a magnetic metal.
As the miniaturization, size reduction and multi-functionalization of cell phones lead to increase in the number of parts, strong demand is mounting on the size reduction of isolators used in cell phones. At present, isolators having outer sizes of 3.2 mm×3.2 mm×1.2 mm and 3.2 mm×2.5 mm×1.2 mm are widely used, but smaller isolators are required. To achieve such size reduction, multilayer ceramic substrates, central conductor assemblies, etc. constituting two-port isolators should be reduced in size.
There are various conventional central conductor assemblies integrally comprising central conductors and ferrite bodies; for instance, those having copper foils wound around a ferrite plate, those having an integrally sintered laminate structure comprising pluralities of ferrite sheets printed with a silver paste to form central conductor patterns (FIG. 14) disclosed in JP 7-212107 A, etc. However, the size reduction of central conductor assemblies to about 1.5 mm×1.5 mm in outer size makes copper foils as thin as about 0.15 mm, vulnerable to breakage, making it difficult to wind central conductors around a ferrite plate at a predetermined crossing angle with secure insulation and high accuracy. On the other hand, the laminated central conductor assembly, which has an integral monolithic structure comprising ferrite and central conductors, is free from the problems of copper foils, but it cannot easily have a large quality coefficient Q, and suffers large resistance, resulting in poor electric characteristics such as insertion loss, etc.